Heavy duty diesel engines generate high cylinder pressures which create a large amount of strain in the engine block during the power stroke of the piston. This strain induces bending loads on the crankshaft connected to the pistons and, in turn, the crankshaft bearings. Typically, engine blocks are formed of a ferrous material, such as cast iron, capable of withstanding the pressure induced strains. However, engine blocks may also be formed of a low density material, such as an aluminum alloy, instead of cast iron, to advantageously increase the power density, i.e. horsepower produced per pound of engine weight. Aluminum or aluminum alloys, while having the advantage of being light in weight, have the disadvantage of possessing less strength than cast iron. In addition, aluminum alloy has a relatively low modulus of elasticity, i.e. less stiffness, compared to cast iron, disadvantageously resulting in excessive deflection and increased strain. The increased strain has been found to result in unacceptably high bending loads on the crankshaft and associated bearings. Moreover, under certain high load applications, cast iron blocks have been found to undergo unacceptable deflection.
Additionally, the loads imposed on the crankshaft during the power stroke are, in turn, transferred to the main bearing caps which secure the crankshaft against the engine block. The main bearing caps are typically bolted to the engine block through threaded portions formed in the bearing saddle. The forces imposed on the main bearing caps can be great enough to generate fatigue cracks in the threaded portion of the bearing saddles possibly resulting in failure of the engine, especially when the threads are formed in an aluminum engine block or other low density material. Thus, in heavy duty diesel engines having high assembly loads in fastening the main bearing cap to the engine block as well as large amount of stress at the threaded capscrew connection of the main bearing to the block during power strokes, it is undesirable to have an aluminum alloy present in the bearing saddle area where the capscrew holes are formed. Moreover, there is still a need for improving the stress resistance of cast iron blocks.
In order to increase the strength of the threaded holes in an engine block, there have been attempts at positioning hardened reinforcements in the bearing saddle area to provide a more secure anchor for the main bearing capscrews. U.S. Pat. No. 4,643,145 to Bolton et al. discloses one such method of reinforcement which includes ferrous reinforcements having threaded bosses for receiving crankshaft bearing cap bolts so as to resist damage to the threads and mitigate forces caused by the engine crankshaft. The ferrous reinforcements are cast into the aluminum block and mechanically held in place by the aluminum casting. The ferrous reinforcement and the aluminum block are chosen to have substantially equal coefficients of thermal expansion thereby overcoming problems caused by differential expansion. However, requiring substantially equal coefficients of thermal expansion limits the possibilities available for the material used for the ferrous reinforcement. Further, the mechanical connection between the reinforcements and the aluminum will inherently contain distinct interfacial discontinuity between the surfaces. The heavy loads imposed on the reinforcements from the bearing cap bolts tend to increase the discontinuities between the surfaces causing the mechanical bond between the reinforcement and the aluminum block to weaken.
U.S. Pat. No. 5,501,529 to Cadle et al. discloses another ferrous insert positioned within the bearing saddle area of an aluminum engine block to support the crankshaft bearing, where the main bearing cap is attached to the ferrous insert by main bearing cap bolts which engage the insert. The ferrous insert is placed into a mold for casting the engine block and molten aluminum is poured around the ferrous insert. A ferrous powder is used to form the ferrous insert so that its surface is not fully dense, allowing the molten aluminum to flow into the holes in the structure of the insert to secure the insert to the engine block. However, this method of attaching the ferrous insert to the aluminum engine block forms a mechanical bond between the ferrous insert and the engine block similar to the bond formed in the '145 Bolton et al. patent. Again, this type of mechanical bond has a tendency to weaken when exposed to the heavy loads imposed on the inserts from the bearing cap bolts.
Another type of bearing saddle reinforcement is disclosed in U.S. Pat. No. 5,370,087 to Guimond et al., where a composite engine including a molded crankcase formed of a lightweight composite material is interconnected with a metallic bearing insert. The metallic insert is attached to a bearing cap so that the crankshaft bearings contact the metallic insert. However, manufacturing tolerances make it difficult for the metallic insert to be attached to the molded crankcase and precisely aligned with the bearing cap for engagement with the bearings. Therefore, an unnecessarily high degree of precision is required when forming the molded crankcase around the insert to ensure proper alignment of the metallic insert with the bearing cap and bearings.
There have also been attempts to improve the bond between metallic inserts and engine blocks to create greater structural integrity. For instance, U.S. Pat. No. 5,333,668 to Jorstad et al. discloses a process for metallurgically bonding an engine cylinder liner insert to an aluminum engine block cast around the insert, where the insert is coated with a metallic bonding material, such as zinc, prior to casting the aluminum engine block. The molten aluminum material then metallurgically bonds to the bonding material coating the insert. Jorstad et al. form the metallurgical bond between the cylinder liner insert and the engine block in order to provide a more continuous bond, thereby allowing for more effective heat transfer from the cylinder liner insert to the engine block. Moreover, cylinder liner inserts are utilized to provided resistance to wear within the cylinder of an aluminum engine block, but are not subjected to large vertical loads such as experienced in the bearing saddle region of the engine block in a heavy duty diesel engine.
In view of the foregoing, there is clearly a need for a reinforcement positioned within the bearing saddle area of an engine block having an improved bond between the reinforcement and the engine block capable of maintaining the bond at high engine loads. Further, there is a need for a hardened reinforcement having this improved bond which allows an engine block to be selectively reinforced in highly stressed areas, of the main bearing saddle region of the engine block.